This invention relates to tennis balls, and, more particularly, to a pressureless tennis ball with an improved core.
Tennis balls conventionally include a hollow rubber core and a felt cover. The core of most new tennis balls is pressurized in order to provide greater rebound. A new pressurized tennis ball conventionally has an internal pressure of about 10 to 15 psi gauge.
Because of the pressure differential between the internal pressure in the core and the ambient pressure, the pressurized gas in the core slowly leaks through the core. As a result, new tennis balls must be packaged in airtight pressurized cans in order to maintain the internal pressure in the balls. However, once the can is opened, the pressurized gas in the core begins to leak through the core. As the internal pressure decreases, the rebound or coefficient of restitution of the ball decreases, and the ball eventually becomes unfit for play.
Pressureless tennis balls are available which have little or no increased internal pressure relative to atmospheric pressure. Pressureless tennis balls do not need to be packaged in a pressurized, airtight container, and the rebound of such balls does not decrease because of leakage of internal pressure.
The specifications of the United States Lawn Tennis Association for tennis balls includes the following:
"The ball shall have a bound of more than 53 inches (135 cm.) and less then 58 inches (147 cm.) when dropped 100 inches (254 cm.) upon a concrete base. PA1 "The ball shall have a forward deformation of more than 0.220 of an inch (0.56 cm.) and less than 0.290 of an inch (0.74 cm.) and a return deformation of more than 0.350 of an inch (0.89 cm.) and less than 0.425 of an inch (1.08 cm.) at 18 lb. (8.165 kg.) load. The two deformation figures shall be the averages of three individual readings along three axes of the ball and no two individual readings shall differ by more than 0.030 of an inch (0.08 cm.) in each case."
The internal pressure of a pressurized tennis ball increases the resilience and rebound of the ball. The core of a pressureless tennis ball is typically designed to be higher in rebound and stiffer than the core of a pressurized tennis ball to compensate for the lack of internal pressure. However, previous attempts to increase the stiffness of a tennis ball core have resulted in a decrease in resilience and rebound of the core, which adversely affects the performance of the tennis ball.